DNA, Structures And Replication Flashcards
How do we know DNA is the molecule of life
Griffith
Avery
Hershey and chase
Experiments in 1920-50
How is DNA structured
Antiparallel strands form double helix
Sugar phosphate backbone
Base pairs join complementary strands by hydrogen bonding
What are the complimentary base pairs of DNA
Adenine with thymine
Guanine with cytosine
What is a nucleotide
Base
Sugar
And phosphate
What are the differences between base pair bonding
2 bonds between adenine and thymine
3 bonds between guanine and cytosine
What is the larger scale packing of DNA
Tightly coiled around histones to form chromatin with tightly coiled ones brain more active genes
How is DNA semiconservative
Each new double stranded molecule contains an original template strand and a newly synthesised complimentary strand
How is DNA synthesised
Starts at Origin of Replication
Utilises dynamic structure of the replication fork
At the replication fork:
Helicases unwind double strand
Single strand binding proteinase stabilise denatured DNA
DNA primase synthesises RNA primer allowing replication to start
DNA polymerase carries out the elongation of the new strand of DNA, formed by the complimentary base pairing to the template strand
What are the two strands of DNA
Leading and lagging strands
What direction does DNA synthesis happen in
5’ to 3’ so requires a free 3’ OH to add on from
What are Okazaki fragments
Short pieces of new DNA made on the lagging strand, sealed by DNA ligase
What do the two types of DNA polymerase do
DNA polymerase 1 replaces the RNA primers with DNA
DNA polymerase 111 adds new nucleotides to the new strands of dna
How are errors in replication amended and what is the resulting error frequency
DNA polymerase has a 3’- 5’ editing function to remove incorrectly inserted bases
This reduces error frequency to around 1 in 10^7
Other enzymes check too and reduce overall error frequency to 1 in 10^9
What causes mutations in DNA
Arise spontaneously due to errors in replication
Induced by DNA damage such as radiation damage or chemical
What are mutagens
Agents which induce mutations
What are the different types of gene mutation
Base substitution
Deletion of bases
Insertion of bases
Rearrangement of bases
What are the consequences of DNA mutation
Can be inherited if in germ cell
In somatic cell, could lead to cancer specifically of in stem cells
How can DNA be repaired
Base excision repair: proteins cut out damaged bases
Nucleotide excision repair: cut out sections of damaged DNA strand
DNA polymerase 1 replaces DNA by copying intact strand and DNA ligase seals it
What is a Barr Body
women inactivate one copy of their X chromosome in each cell and it is pushed to the edge of the nucleus
What is synteny
Where long strands of DNA sequences are present in the same order across species
What is translocation and what can it cause
Chromosome breakage and reforming
Can cause: disease, cancer, developmental abnormalities and can be inherited
What does Xist do?
A regulatory RNA that switches off a copy of the X chromosome in XX cells
What are exons and introns
Exons are coding sections of pre-mRNA
Introns are regions which are spliced out of pre-mRNA
What are genes used for
Transcribed into Structural RNA or regulatory RNA
Transcribed into mRNA which is translated into proteins
May not contribute to making us, but are remnants of past events
What is a retrovirus
RNA virus which infect cells and have reverse transcriptase which allows them to revert to DNA and join with our genome upon infection
What are pseudogenes
Sequences that are common with functional human genes but are non-functional
What are VNTRs
Variable number tandem repeats
Short sequences which are repeated multiple times
What are SNPs
Single nucleotide polymorphisms
What is a polysome
An RNA molecule that is covered in ribosomes
What are lysosomes
Full of degradative enzymes which degrade certain molecules imported into the cell and old or damaged cellular components
What do the ER and Golgi apparatus do
Various processes including the packaging and secretion of proteins
What are the characteristics of mitochondria
The sites of energy generation in the cell by burning fuel to make ATP
Small circular chromosome inside them
Inherited mitochondrial DNA from mother
What is a TATA box
Promoter region to initiate transcription
Short run of thymine and adenine
25 base pairs before start of transcription
How is transcription initiated
Promoter region -> transcription factors bind -> RNA polymerase II binds-> some transcription factors disassociate -> transcription begins
What is the difference between eukaryotes and prokaryotes
No nucleus in prokaryotes
Prokaryotes have polysistzonic rna which can code for multiple proteins in one strand of RNA sequence
What does the 5’ cap do
Protects the mRNA from degradation
Helps with transcription
How does a eukaryotic mRNA look
5’ cap -> non coding sequence -> coding sequence -> non coding sequence -> polyA tail
What is polyadenylation
Transcription continues past the point where one or more poly A sequences are present, then stops
mRNA is then cut near the poly A sequence and a poly A tail is added
What is splicing
Removal of intronic sequences from pre-mRNA
What are locus control regions
Isolated promoter regions far away from the desired gene
What are the three stop codons
UAA
UAG
UGA
What is the start codon
AUG methionine
What forms the Rough Endoplasmic Reticulum
membranes with bound ribosomes
What are the two types of ribosomes
cytosolic
membrane-bound
How is the translated sequence signalled to be a secretory protein
first part of sequence is a stretch of around 20 hydrophobic amino acids
this is a signal sequence
Why do ribosomes then bind with to the endoplasmic reticulum
signal recognition particles bind to the signal sequence, causing the ribosome to dock to the ER
What its the result of ribosome binding to the ER
the newly synthesised protein is fed through a channel into the lumen of the ER as it is translated,
the amino terminal signal sequence is usually cut off
What happens with transmembrane proteins
Translation/ translocation continues until a stretch of ~20 consecutive hydrophobic amino acid is made ( a transmembrane domaine) This is embedded in the membrane, The ribosome continue to make the rest of the membrane in the cytosol
What happens in the ER
Disulphide bridges form
Sugar side chains added
What happens in the Golgi apparatus
Some ER sugars are trimmed and replaced
How are mitochondrial proteins made
deposited in cytosol
post-translational import
Chaperone binds to protein to prevent folding allowing it to be posted into mitochondria
mitochondrial targeting sequence at end of protein to promote import into mitochondria
What are TOM and TIM of the mitochondria
Two channels which the protein destined for mitochondrial matrix is threaded through
Translocase of the outer membrane (TOM)
Translocase of the inner membrane (TIM)
What is cancer
A group of diseases
Solid tumours and leukaemias
Disease of body’s own cells
What is are key features of cancer at the cellular level
uncontrolled cell division
change in morphology
dedifferentiation of cells
Cell migration into adjacent and distant tissues
What is the progression through cancers for cells
Normal cells -> hyper proliferative cell population -> early adenoma -> late adenoma -> carcinoma
What are the three types of environmental factors that can cause cancer
Biological
chemical
physical
What does an epidemiology study show
That a chemical is carcinogenic in human
Looks for an association between the exposure and cancer
